This invention relates to a signal intercepting circuit and particularly, to a muting circuit for intercepting an audio signal supplied to a loudspeaker in an audio amplifying circuit in a radio, such as a stereophonic receiving apparatus, or a television receiver.
In an audio amplifying circuit, a muting circuit is used to temporarily prevent an audio signal from being supplied to the loudspeaker when a power supply switch is turned on, or when the band selecting or STEREO MONO selecting switches is changed.
A muting circuit of the prior art is shown in FIG. 1. An audio signal is supplied to an input terminal 11. Terminal 11 is connected, through a resistor R.sub.11, to the base electrode of a transistor Q.sub.11. The base electrode of transistor Q.sub.11 is also grounded through a resistor R.sub.13. Its collector electrode is grounded and its emitter electrode is connected to the emitter electrode of a transistor Q.sub.12. The collector electrode of transistor Q.sub.12 is connected to an output terminal 14 through an amplifier 12. The base electrode of transistor Q.sub.12 is connected, through a negative feedback resistor R.sub.12, to output terminal 14 and to ground through serially connected condenser C.sub.11 and resistor R.sub.14. Output terminal 14 is connected to a loudspeaker (not shown). Further, the collector electrode of a muting transistor Q.sub.13 is connected to the base electrode of transistor Q.sub.11, while its emitter electrode is grounded. A muting pulse is supplied to the base electrode of transistor Q.sub.13 in response to, for example, the turning on of a power supply switch (not shown).
The operation of the muting circuit is as follows: when a positive muting pulse is supplied to the base electrode of transistor Q.sub.13, transistor Q.sub.13 is turned on. Accordingly, the audio signal is bypassed to the ground through transistor Q.sub.13, rather than being supplied to the base electrode of transistor Q.sub.11.
At the transient time transistor Q.sub.11 is turned on, its base potential, however, drops from I.sub.B R.sub.B to zero volts. As a result, a large AC pulsed waveform is produced at output terminal 14 which is thereby supplied to the loudspeaker. This pulsed AC waveform causes the loudspeaker to produce a "pop" sound. I.sub.B is the base current of transistor Q.sub.11 and R.sub.B is the resistance of resistor R.sub.13.
In order to prevent the base potential of transistor Q.sub.11 from dropping, a condenser (not shown) must be connected between the collector electrode of transistor Q.sub.13 and the terminal of resistor R.sub.13 which is connected to the base electrode of transistor Q.sub.11. Since the capacitance of such a condenser must be considerably large, the resulting muting circuit has the disadvantage that it cannot be fabricated into a semiconductor integrated circuit.